Machining such as a heat treatment including cutting, perforation, welding, cladding buildup and quenching has come to be carried out by irradiating a laser beam to a machining-target site of a work piece. In these various kinds of laser machining, in general, a YAG laser beam and a carbon dioxide gas laser have conventionally been employed.
When a YAG laser or a carbon dioxide gas laser is employed, in order to obtain an energy density and output corresponding to the laser machining, the emitted laser beam is reduced into a predetermined spot diameter by use of a condenser lens or a condenser reflecting mirror and then is irradiated to the machining-target site. Especially in welding, cladding buildup, quenching and the like where the machining-target site Wa in a predetermined range of a work piece W is machined without turning its state into a gaseous phase, a laser beam L with a reduced diameter is irradiated to the machining-target site Wa in the predetermined range of the work piece W. In addition, for such purpose as to prevent the machining-target site Wa from burn through or penetration and bulging at a center portion, scanning or oscillation is carried out (hereinafter, generally referred to as oscillation) in a state where the laser beam L is vibrated by vibrating a condenser lens 3 as shown in FIG. 21 or repetitively rotating a scanning mirror 4 as shown in FIG. 22.
FIG. 25 shows a case where cladding buildup in which a buildup material such as powder metal is deposited onto a base material is carried out by employing a conventional technique. A laser beam is irradiated to the buildup material at a uniform output. Further, FIG. 26 shows a case where aluminum plated steel sheets in each of which aluminum is plated onto a base material made of steel sheet are laminated and welded with each other in the state where their aluminum plated layers are attached to each other, by employing a conventional technique. In addition, FIG. 27 shows a case where zinc plated steel sheets in each of which zinc is plated onto a base material made of steel sheet are laminated and welded with each other in the state where their zinc plated layers are attached to each other, by employing a conventional technique.
Further, in recent years, as is disclosed in Japanese Patent Publication No. 2683158, various kinds of machining has come to be carried out by use of a semiconductor laser system. The laser system disclosed in Japanese Patent Publication No. 2683158 is a semiconductor laser system including a plurality of semiconductor laser units each having a laser oscillator. The semiconductor laser units generate laser radiation beams emitted therefrom. Each of the semiconductor laser units includes a plurality of groups of semiconductor laser units. The semiconductor laser units in each of the groups operate in the same basic mode in a horizontal direction and in the same mode in the longitudinal direction in order to generate laser radiation beams with the same wavelength, have an photoconduction single mode related to each of the semiconductor laser units, and have a connection element for connecting the laser beam emitted from each of the semiconductor lasers with one of the single-mode photoconduction fibers, respectively. The photoconduction fibers form a fiber bundle having an end surface, and the end surface includes fiber end surfaces which form the fiber bundle in order to render all of the fiber end surfaces that generate laser radiation beams with the same wavelengths into a radiation group. The radiation groups which generate laser radiation beams with different wavelengths from each other are then arranged on the end surface. A coherent laser radiation generated by each of the semiconductor laser units and emitted from the front end surface of the fiber bundle forms the entire laser radiation. The entire laser radiation irradiates a certain target surface of an object which is irradiated while all the semiconductor laser units are in operation, and a controller is provided so as to control each of the semiconductor laser units in a determined state. Since the light intensity of irradiation can be specified for each of the surface elements, it is possible to determine irradiation for each of different surface elements on the target surface.
Further, this Patent Publication discloses, for example, two irradiated surface portions having unique shapes which operate for pre-heating during alloy machining or for post-treatment, and an oval-shaped surface portion extending in parallel to the movement direction toward the axis in a length direction.
In other words, the laser system of this Patent Publication performs an ON/OFF control for each of the semiconductor laser units in order to specify the laser radiation in correspondence with each of the different surface elements on the target surface.
However, among the aforementioned conventional techniques, when specified machining is performed in the state where the laser beam L is oscillated within a specified range by employing a YAG laser or a carbon dioxide gas laser, precise control is required for vibrating the condenser lens 3 (FIG. 21) and for repetitively rotating the scanning mirror 4 (FIG. 22). In addition, since it is required to provide the laser machining apparatus with a mechanism for vibrating the condenser lens 3 or a mechanism for repetitively rotating the scanning mirror 4 (its illustration is omitted), the apparatus becomes complicated and large-sized as a whole, and maintenance for maintaining these mechanisms is also required. Further, the vibration given to the condenser lens and the repetitive rotation of the scanning mirror 4 make it difficult to enhance the durability of the laser machining apparatus.
Further, when cladding buildup is carried out as shown in FIG. 25, since the heat is hard to be released at a center area of the molten portion of the base material, the molten portion tends to grow large. On the contrary, the end portion thereof is cooled down easily because the heat is easily released. Therefore, if a laser beam is irradiated to the buildup material at a uniform output as described above, a difference in temperatures is caused. In this case, at the center area where the molten portion grows to be large, the base material is diluted and the composition of the buildup material changes. Especially in the case where the base material is made of copper and the buildup material is made of aluminum, if the base material is diluted and the composition of the buildup material changes, there arises a problem that the hardness increases and cracks are created in accordance with the difference in temperatures. The problem resulted from the difference in temperatures from portion to portion of the machining-target site due to the irradiation of the laser beam at a uniform output as described above arises not only in the cladding buildup but also in the heat treatment such as quenching and other types of machining.
Further, when the aluminum plated steel sheets are laminated and welded with each other as shown in FIG. 26, the aluminum plated layers in contact with each other are melted and an intermetallic compound of aluminum and iron is produced at the molten portions. Since the intermetallic compound thus produced has high hardness, there arises a problem that the molten portions become brittle.
Furthermore, when the zinc plated steel sheets are laminated and welded with each other as shown in FIG. 27, since the zinc plated layers have a low melting point, they are evaporated to produce bubbles. In this case, there arises a problem that the bubbles blow off at the rear of the welding pool in the machining direction (see the arrow in FIG. 27) and blowholes are created in the weld bead.
Meanwhile, in the laser system disclosed in the Patent Publication No. 2683158, laser radiation is merely emitted in correspondence with the different surface elements on the target surface by simply carrying out the ON/OFF control for each of the semiconductor laser units. When the work piece W is machined by changing the state of the machining-target site Wa of the work piece W into a liquid phase without oscillation such as that performed in the case where the YAG laser or the carbon dioxide gas laser is employed as mentioned above, the machining-target site Wa is melted without being stirred and then is re-solidified. Therefore, as shown in FIGS. 23 and 24, a delay G in re-solidification is partially produced at the rear in the machining direction. In this case, a crack C similar to a shrinkage cavity or a blowhole may be produced in the bead Y. For this reason, the laser system disclosed in this Patent Publication also requires an installation of a laser oscillator to the semiconductor laser units, as is the aforementioned case where the YAG laser or the carbon dioxide gas laser is employed. In this case, precision is required for control, and the system becomes complicated and large-sized, and maintenance is required. In addition, it is difficult to increase the durability of the system.
The present invention has been made in view of the problems described above, and an object thereof is to provide a laser machining apparatus in a simple structure which can properly carry out laser machining by irradiating a laser beam easily and precisely at a predetermined energy density and output as well as in a predetermined time duration to a machining-target site in a predetermined range, and in addition, which can be downsized and can be kept with easy maintenance and has enhanced durability.
In addition, another object of the present invention is to provide a method for properly carrying out laser machining by irradiating a laser beam at a predetermined energy density and output as well as in a predetermined time duration to a machining-target site in a predetermined range in a simple control.